Parking devices for use in transmission devices have heretofore been developed, each including: a piston unit that is placed so as to be movable in the axial direction thereof in a housing and is operated, according to a fluid pressure, in the direction of releasing a parking lock against a spring device acting in the direction of applying the parking lock; a latch mechanism that includes three spring arms and is capable of holding the piston unit in a predetermined position in the axial direction thereof; a tripping member that is movable together with a piston rod in the housing and is urged toward the piston unit by a spring; and an electromagnetic operating device that is arrange coaxially with the piston unit and operates the tripping member (refer to Patent Document 1, for example).
In such a parking device, when the piston unit is being hydraulically moved toward the tripping member, the tripping member pressed by the piston unit moves toward the electromagnetic operating device against an elastic force (urging force) of the spring, and the piston unit engages with the latch mechanism. In this state, the parking lock is released, and the parking lock released state can be maintained by energizing the electromagnetic operating device to fix the position of the tripping member. When the electromagnetic operating device stops being energized while the hydraulic pressure is not supplied to the piston unit, the elastic force of the spring moves the tripping member toward the piston unit to push to expand the three spring arms, and releases the engagement between the latch mechanism and the piston unit. When the piston unit is being moved by an elastic force of the spring device or a hydraulic pressure so as to be away from the electromagnetic operating device, the parking lock is applied.
Moreover, locking units for use in parking brakes of automobiles have been proposed, each including: a piston that is placed in a housing and is hydraulically movable in the direction of releasing a parking lock; a shock-absorbing spring placed in an opening on a distal end side of the piston; an operating element that is capable of holding the piston by engaging with a catching element provided at the distal end of the piston; a bolt fixed to an iron core bar connected to an iron core; a spring for urging the bolt toward the piston; and a coil, a yoke, and a case for holding the position in the axial direction of the iron core (refer to Patent Document 2, for example).
In such a locking unit, when the piston is being hydraulically moved toward the bolt, the shock-absorbing spring in the opening of the piston abuts on the iron core bar, and the catching element of the piston engages with the operating element. At this time, the parking lock is released. The parking lock released state can be maintained by energizing the coil to attract the bolt toward the coil side at this time. When the coil stops being energized while no hydraulic pressure is applied to the piston, the urging force of the spring moves the bolt toward the piston to push to expand the operating element, and releases the engagement between the catching element of the piston and the operating element. The piston moves away from the bolt to apply the parking lock.